(Supported by the NIH/NCRR/P41 RR 01219 grant). Controlling the temperature of living specimens throughout an experiment, and while viewing by video time-lapse microcopy, is fundamental to maintaining their viability. In addition, in many cases an experimental protocol calls for rapidly cooling and then re-warming a cell while it is being viewed by high resolution video-light microscopy. For example, in certain mammalian cells all microtubules within the cell can be disassembled by rapidly cooling it to 6oC, and re-warming then allows the microtubules to re-grow from the centrosome. In some cases it is desirable to be able to de-polymerizes the microtubules in a specific cell, and then while viewing the cell destroy its centrosome by laser microsurgery or manipulate it by optical trapping. To this end we have designed and constructed an inexpensive Peltier-driven cooling and heating microscope stage. The stage can cool the specimen within a Rose or perfusion chamber (even when viewed with oil immersion objectives) to ~ 4xC in less than 6 minutes, hold it at this temperature for as long as needed (>60 min), and then when desired warm the specimen to homeostatic temperature. The Peltier device allows for the rapid cooling/heating due to the large (T that can be generated between the device and the specimen. This characteristic, when coupled to the reversibility of heat transfer function and the low cost of the Peltier devices themselves (<$20.00 ea.) makes this an ideal temperature control stage for experiments on the stage of a light microscope that call for rapid, controlled and sustained temperature shifts. Rieder, C. L. and R.W. Cole. 1998. Perfusion chambers for high-resolution video-light microscopic studies of vertebrate cell monolayers: some considerations and a design. In "Video Microscopy", G. Sluder and D. E. Wolf, Eds. Methods in Cell Biology 56:253-275.